The present invention relates to an indicating instrument of the cross-coil type and, more particularly, to a cross-coil type indicating instrument suitable for use in a vehicle such as an automobile to serve as an on-vehicle indicating instrument of various kinds, such as a speedometer, tachometer or boost meter.
Hitherto, a type of on-vehicle indicating instrument having a pair of coils for generating magnetic fields perpendicular to each other has often been employed. Electric current changing with changes in the quantity to be measured is passed through the coils, and a magnet rotor is rotated in the direction of a combination magnetic field which is the combination of the respective magnetic fields generated by the pair of coils. A pointer which rotates together with the magnet rotor indicates the measured quantity.
A conventional indicating instrument of this type has a coil bobbin on which coils are wound in such a manner as to be perpendicular to each other. A disk-shaped magnet rotor with S and N poles formed thereon is disposed inside the coils and the coil bobbin. The magnet rotor has a rotary shaft which is integrally mounted thereon and which extends through the center of the rotor.
The coil bobbin is divided, at a position substantially intermediate thereof, into two coil bobbin sections which are adjacent to each other longitudinally of the rotary shaft. A first coil bobbin section has a first bearing hole formed therein, and a first end portion of the rotary shaft which projects from the magnet rotor is rotatably supported by the first bearing hole for sustaining radial load. A second coil bobbin section has a second bearing hole formed therein, and a second end portion of the rotary shaft is rotatably supported by the second bearing hole for sustaining radial and thrust loads. A recess filled with silicone oil, serving as a viscous fluid for damping, is formed in the periphery of the second bearing hole, and the recess serves as an oil reservoir.
The conventional instrument further has a damping projection formed integrally with the magnet rotor at a lower position of the rotor. The damping projection is dipped in the silicone oil received in the oil reservoir. With the damping projection being dipped in the silicone oil, the following effect is provided: when the magnet rotor and the rotary shaft integral with the rotor rotate together, their rotation inevitably requires that the damping projection also rotate against the viscosity of the silicone oil. Therefore, the rotation of the magnet rotor and the rotary shaft is damped by the silicone oil and the damping projection, and this damping serves to prevent unnecessary vibration of the rotary shaft.
However, the above-described construction of the conventional indicating instrument entails the following problem. In said construction, the oil reservoir is formed in the periphery of the second bearing hole acing as a thrust bearing means for the rotary shaft, and the damping projection of the magnet rotor is dipped in the silicone oil in the oil reservoir. However, when the indicating instrument is installed in a vehicle, and is thus used in an environment in which the instrument can be subjected to vibration caused by vehicle running, there is a risk that the magnet rotor may be displaced in the thrust direction of the rotary shaft. Upon such displacement, the displacement of the magnet rotor causes a corresponding reduction in the surface area of that portion of the damping projection dipped in the silicone oil in the oil reservoir, thereby causing a reduction in the damping force acted on the damping projection by the silicone oil. In this way, the damping force, which is required in the very event of vibration, is diminished, ending in unstable damping. Thus, with the conventional instrument, it has been difficult to effectively restrain unnecessary oscillation of the pointer caused by vibration.